The reproductive system, as in the two preceding orders, consists of a vertical pillar, the "genital stolon," and a circular "genital rachis" giving off interradial branches from which the genital organs bud. The genital stolon is developed from the wall of the general coelom near the upper end of the axial sinus; it attains a great development and ultimately completely surrounds the axial sinus, which then appears like the cavity of a glandular tube, the walls of which are constituted by the genital stolon. The compound structure consisting of stolon and axial sinus was actually described as a nephridium by the Sarasins[[486]] in the case of Asthenosoma. Its true nature, however, is shown when the upper end is examined; it is then seen to open into the stone-canal and to be in communication with the ampulla, into which the pore-canals open. Lying alongside the upper end of the axial sinus is the somewhat elongated "madreporic vesicle," or right hydrocoele, which was described by Sarasin as the accessory kidney (Nebenniere), since like the axial sinus it is partly enveloped by the genital stolon. Leipoldt,[[487]] however, showed clearly that it is a completely closed space.
The genital rachis springs from the upper end of the stolon, and as in Asteroids, it lies in the outer wall of a space called the "aboral sinus" (Fig. 234, 20) intervening between it and the test. In adult specimens it seems to degenerate. The genital organs are situated at the ends of five interradial branches of the rachis (Fig. 231, gon). Each is an immense tree-like structure consisting of branching tubes, which are lined by the sexual cells. So enormous do they become in the breeding season that they form an article of food among fishermen. The term esculentus is derived from this circumstance. Other species are regularly sold for food as Frutta di Mare (Fruit of the Sea) at Naples, and as "sea eggs" in the West Indian Islands. One female Echinus esculentus will produce 20,000,000 eggs in a season.
The so-called blood system is more distinctly developed in Echinoidea than in Asteroidea and Ophiuroidea. There is an oral ring of lymphoid tissue surrounding the oesophagus below the water-vascular ring. From this are given off two strands, the so-called "dorsal" (Fig. 231, b.v), and "ventral" vessels (Fig. 234, 16), which run along the two opposite sides of the stomach or first coil of the alimentary canal. The position of these strands suggests that like the lacteals of the human intestine they are channels along which the products of digestion exude from the stomach. The dorsal strand is situated on the same side as the genital stolon, and from it branches are given off which ramify on the surface of the stolon, on account of which this organ, as in Asteroidea, was at one time regarded as a "heart," but the distinction of the stolon from the strands is easily made out. An aboral ring enclosing the genital rachis lies embedded in the septum dividing the aboral sinus (Fig. 234, 20) from the general coelom.
Classification of Echinoidea.
The Echinoidea are sharply divided into three main orders, which differ from each other profoundly in their habits and structure. These are: (1) The Endocyclica or Regular Urchins, of which the species just described may be taken as the type. (2) The Clypeastroidea or Cake-urchins, which are of extremely flattened form, and in which the periproct is shifted from the apical pole so that it is no longer surrounded by the genital plates, while some of the tube-feet of the dorsal surface are flattened so as to serve as gills. (3) The Spatangoidea or Heart-urchins, in which the outline is oval: the periproct is shifted, as in the Cake-urchins, and the dorsal tube-feet are similarly modified; but the Heart-urchins have totally lost Aristotle's lantern, whilst the Cake-urchins have retained it. This strongly-marked cleavage of the group was primarily due, as in all such cases, to the adoption of different habits by different members of the same group. Were we to term the three orders Rock-urchins, Sand-urchins, and Burrowing-urchins, it would not be entirely true, for secondary invasions of the other's territory on the part of each order have undoubtedly taken place; but still the statement would remain roughly true, and would give a fair idea of the differences in habitat which have led to the differentiation of the group.
Order I. Endocyclica (Regular Urchins).
The principal variations concern (1) the peristome, (2) the periproct, (3) the corona, (4) Aristotle's lantern and its appendages, (5) the spines, (6) the pedicellariae, and lastly, (7) the tube-feet. We shall consider these points in order.
Peristome.—In the vast majority of species this region is covered only with flexible skin in which ten small plates are embedded, pierced by pores for the buccal tube-feet; besides these there are irregularly arranged thin plates. In the Cidaridae both the ambulacral and the interambulacral series of plates are continued on it; these plates differ from those of the corona in being movable on one another. In Echinothuriidae only the ambulacral series of plates is continued on to the peristome. In the case of both these families there are a considerable number of tube-feet within the region of the peristome which may be classed as buccal.
Periproct.—This area, which represents the whole dorsal surface of Asteroidea, is very large in the Cidaridae, where, as in Echinus, it is covered with leathery skin in which small plates are embedded. In the Saleniidae it is covered with a single large sur-anal plate, in the edge of which the anus is excavated; in the Arbaciidae it is covered with four valve-like plates; whilst in the remaining species its condition is similar to that described in the case of Echinus esculentus.
Corona.—In Echinothuriidae all the plates are separated by slips of membranous skin, so that the test is flexible. In all other families it is an unyielding cuirass. In the Cidaridae the pore-plates remain separate throughout life, and are therefore identical with the ambulacral plates. These are small and placed in two vertical rows, and so the ambulacra are exceedingly narrow. In Echinothuriidae there is some tendency to adhesion amongst the pore-plates; these are of different sizes, and usually one larger and one smaller adhere to one another. In all other species regular ambulacral plates are formed at least in the lower part of the radii near the peristome by the adhesion of the pore-plates in groups of two, three, or more. Sometimes as many as nine pore-plates may thus adhere.